1. Field of Invention
This invention relates generally to baking utensils, and more particularly to a utensil adapted to operate in a microwave oven to convert microwave energy to which the utensil is exposed to thermal energy for cooking the food contained therein.
2. Status of Prior Art
Baking is the process of cooking food with dry heat. Heat is transferred to the food by convection, conduction or infrared radiation, depending on the nature of the oven. But in all cases, thermal energy is applied to the outer surface of the food and by reason of heat transfer from this surface to the inner body of the food, cooking takes place from the outside in. As a consequence, as cooking continues, the surface of the food may become scorched or browned. Because this action usually renders the food more palatable, such surface scorching or browning is normally regarded as desirable.
The use of a microwave oven to heat or cook food is commonplace, and microwave ovens are now installed in many households. In the typical microwave oven, a magnetron functions to generate microwave energy at a frequency of about 1000 mHz. This energy is conveyed by a wave guide to the interior of the oven to irradiate the food placed therein. Because food more or less absorbs microwave energy, this gives rise to internal molecular friction which heats the food at a rate that depends on its "lossy" characteristics. Some food products are heated more rapidly than others in a microwave oven; but in general the cooking of food by microwave energy is much faster than by conventional heating techniques, including infrared radiation.
In heating or cooking food in a microwave oven, the food is placed in a receptacle of synthetic plastic, glass or other material which is non-reactive to microwave energy; hence, it is only the food that is heated. U.S. Pat. Nos. 4,703,149 and 4,416,906 disclose microwave food heating containers. In some cases, as pointed out in U.S. Pat. No. 4,416,906, microwave cooking of food is uneven because of dry spots in some areas of the food and moist spots in other areas.
Cooking takes place in a microwave oven throughout the body of the food; hence browning or scorching of the outer surface or crust does not occur. Thus if one sought to bake a loaf of bread in a microwave oven it would have no crust. This is a recognized disadvantage of microwave cooking.
U.S. Pat. No. 3,941,967 discloses a microwave cooking apparatus capable of scorching the surface of the food being cooked without excessively heating the interior of the food. This apparatus, which is designed to be put into a microwave oven, is in the form of a casing within which is disposed a plate on which the food to be cooked is placed. Below the plate is a thermal heating element which generates heat by absorption of microwave radiation, use for this purpose being made of a ferrite ceramic. Thus, the interior of the food is heated by the microwave energy absorbed thereby, while at the same time the exterior of the food is thermally heated and scorched by the plate heated by the ferrite heating element.
A similar arrangement is shown in U.S. Pat. No. 4,496,815, in which a microwave browning utensil includes a metal platter on which the food to be heated in the microwave oven is placed. On the underside of the platter is a thermal heating element formed by powdered ferrite dispersed in a matrix of organic material. In this way, the interior of the food on the platter which absorbs microwave energy is heated and cooked, while its exterior is thermally heated and browned. Thermal heating takes place mainly by conduction; hence, the exterior of a body being heated is first subjected to the heat before it penetrates the interior of the food body, whereas with microwave heating, the radiation penetrates the interior of the body.
U.S. Pat. No. 4,266,108 discloses the use of ferrite material adjacent a microwave reflecting member in which the ferrite material acts as a heating element that will rise in temperature to a predetermined level which depends on the Curie point of the ferrite.
The Curie point of a ferrite is the temperature value marking the transition between ferromagnetism and paramagnetism. When in its ferromagnetic state, the ferrite then absorbs microwave energy and is heated thereby. This action ceases when the ferrite enters its paramagnetic state. Hence, when a ferrite heating element is placed in a microwave oven and is subjected to microwave energy, the heating element will become increasingly hot until an elevated temperature is reached that depends on the Curie point of the ferrite, after which no more heat is generated even though the microwave oven is still operating. Thus, the ferrite heating element will effectively be turned "off," even though the microwave oven is still "on."
In the above-noted prior art arrangements which make use of ferrite heating elements, cooking is effected primarily by means of microwave energy, the thermal energy generated by the microwave-absorbing ferrites serving only to brown or scorch the surface of the food. Hence cooking takes place effectively from the inside out except for the thermal energy applied only to the outer surface of the food in the relatively brief period in which microwave cooking is in progress.
In other words, if in order to cook a particular food product with microwave energy in a microwave oven, the microwave cooking time is 5 minutes, and the microwave oven timer is so set, then at the end of 5 minutes, the microwave-absorbing thermal energy element will be turned off even though browning of the food has not yet been effected.